get a list of all newly created/edited files in a github pull request from a bash script

I have a github actions set up to run on the creation/synchronization of pull requests. I want to be able to get a list of all the JSON filepaths that were either created, or edited in the pull request, and do something with this list of filepaths.

Github Workflow

name: PR
on:
  pull_request

jobs:
  run-script:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v3
      - name: run the script
        run:./.github/scripts/script.sh
        shell: bash

script.sh

for filepath in $(...); do # get the list of filepaths that were created or edited in the PR
  # do something with the filepaths
done

My attempt

for file in $(git diff-tree --no-commit-id --name-only -r HEAD~1 HEAD | grep '\.json$'); do
done

but this gave me an error

fatal: ambiguous argument 'HEAD~1': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

For the script, you can get the diff of json files like so:

mergeBranch='main' # Change if PR merges into a different branch
oldCommit=$(git log $mergeBranch | head -n 1 | awk -F ' ' '{print $2}')
latestCommit=$(git log | head -n 1 | awk -F ' ' '{print $2}')
git diff --merge-base $oldCommit $latestCommit -- '*.json'

Also, this is not a requirement, but a suggestion to edit less files. You can have the contents of the script inside of the GitHub Action step itself as so:

  - name: Check Diff
        run: |
           # Script Contents Below